Method for determining a scale inhibitor concentration in a sample

ABSTRACT

The invention relates to a method for determining a scale inhibitor concentration in a sample comprising at least a first scale inhibitor, which is a synthetic organic compound comprising at least one ionised group. The method comprises optionally diluting and/or purifying the sample, and allowing the sample to interact with a reagent comprising a lanthanide(III) ion. The sample is excited at a first excitation wavelength and a sample signal deriving from the lanthanide(III) ion is detected at a signal wavelength by using time-resolved luminescence measurement, and the concentration of the at least first scale inhibitor in the sample is determined by using the detected sample signal.

This application is a continuation application of U.S. Ser. No.15/037,680, which is a US national application of internationalapplication PCT/FI2014/050877, filed on Nov. 18, 2014, and claimspriority of Finnish national application FI20136151 filed on Nov. 19,2013, contents of all of which are incorporated herein by reference.

The present invention relates to a method for determining a scaleinhibitor concentration in a sample according to the preambles of theenclosed independent claims.

Scale inhibitors are used, for example, in offshore oil production forstimulation of the oil wells, for controlling and/or preventing scaledepositions. A scale inhibitor may be injected continuously into an oilwell, or it may be periodically injected if a so-called squeezetreatment is employed. In the squeeze treatment a scale inhibitor pulseis injected into the oil well and the scale inhibitor leaches back intothe produced fluids. The concentration of the scale inhibitor in theproduced fluids should be sufficiently high in order to avoid scaleformation or precipitation. The concentration of scale inhibitornormally decreases exponentially after the initial injection, and whenthe concentration has fallen below a predetermined value the squeezetreatment of the oil well is repeated. Consequently, it is important toobtain reliable knowledge about the concentration of the scale inhibitorin the produced fluids for securing well-timed squeezing treatment. Ifthe squeezing treatment is performed too late, harmful scales may beformed and disturb the production process.

Nowadays different analytical techniques are used for determining thescale inhibitor concentration in the produced fluids. Examples of usedtechniques are inductively coupled plasma (IPC), high-performance liquidchromatography (HPLC) and liquid chromatography-mass spectrometry(LC-MS). However, there is a continuous need for new, accurate andsimple analysis methods.

It is an object of the present invention to reduce or even eliminate theproblems appearing in prior art.

An object of the invention is to provide a simple and reliable methodfor determining the amount of scale inhibitor in a sample, especially inan oilfield sample.

Another object of the present invention is to provide a fast method fordetermining at least one scale inhibitor in a sample.

In order to realise the above-mentioned objects, among others, theinvention is characterised by what is presented in the characterisingparts of the enclosed independent claims.

Some preferred embodiments according to the invention are disclosed inthe dependent claims presented further below.

Typical method according to the present invention for determining ascale inhibitor concentration in a liquid sample comprising at least afirst scale inhibitor, which is a synthetic organic compound comprisingat least one ionised group, comprises

-   -   optionally diluting and/or purifying the sample,    -   allowing the sample to interact with a reagent comprising a        lanthanide(III) ion,    -   exciting the sample at a first excitation wavelength and        detecting a sample signal deriving from the lanthanide(III) ion        at a signal wavelength by using time-resolved luminescence        measurement, and    -   determining the concentration of the at least first scale        inhibitor in the sample by using the detected sample signal.

The method according to the present invention is suitable fordetermining concentration of scale inhibitor in any industrial watersystem or industrial water system sample. These industrial water systemswhere scale inhibitors may be employed include, but are not limited to,cooling tower water systems including open, recirculating, closed andonce-through systems; petroleum wells, downhole formations, geothermalwells and other oil field applications; boilers and boiler watersystems; mineral process waters including mineral washing, flotation andbenefaction; paper mill digesters, washers, bleach plants and whitewater systems; black liquor evaporators in the pulp industry; gasscrubbers and air washers; continuous casting processes in themetallurgical industry; air conditioning and refrigeration systems;industrial and petroleum process water; indirect contact cooling andheating water, such as pasteurisation water; water reclamation andpurification systems; membrane filtration water systems; food processingstreams, such as meat, vegetable, sugar beets, sugar cane, grain,poultry, fruit and soybean processing streams; and waste treatmentsystems as well as in clarifiers, liquid-solid applications, municipalsewage treatment and industrial or municipal water systems. Preferablythe method is used for determining a concentration of at least one scaleinhibitor in a sample originating from petroleum wells, downholeformations, geothermal wells and other oil field applications. Whilesome of the exemplary methods are described herein in relation to asample originating from an oilfield or an oil well or from an oilproduction process, it will be understood that the methods may beadapted for use with other such samples and/or systems.

Now it has been surprisingly found out that by using the exemplarymethods described herein the time-resolved luminescence signal derivedfrom an interacted reagent comprising a lanthanide(III) ion, such aseuropium, excited at a suitable first wavelength, correlates accuratelyto the scale inhibitor concentration in a sample. This method may beutilised for determining the presence and/or concentration of a scaleinhibitor in the sample and it can detect even low scale inhibitorconcentration in a sample, such as an oilfield sample. Significantreduction in the detection limit of scale inhibitor concentrations maybe achieved by using time-resolved luminescence signal of alanthanide(III) ion. The detected sample signal normally increases inthe presence of a first scale inhibitor and correlates to theconcentration of the first scale inhibitor in the sample. A furtheradvantage is that the method according to the invention is simple andfast to perform.

As used herein the term “scale inhibitor” is used in its ordinary senseas understood by one skilled in the art, and thus may be used herein torefer to or describe synthetic chemical compositions or syntheticorganic compounds, which comprise at least one ionised group, and which,when added to an aqueous system that tends to form scale, reduce,control, disperse or inhibit the formation, deposition and/or adherenceof scale deposits on substrate surfaces in contact with a scale-formingaqueous system. In the context of the exemplary embodiments the term“scale inhibitor” denotes a synthetic organic compound or substance,preferably a synthetic polymer or copolymer.

As used herein the term “polymer” is used to denote a syntheticsubstance which is composed of a number of repeating monomer units, sameor different, joined together to form a polymer backbone. A polymer isformed of at least two, preferably a plurality of monomers. As usedherein the term “copolymer” is used to denote a polymer which comprisestwo or more different monomer units. The type of the copolymer dependson the arrangement of the different monomer units in its structure. Thecopolymer may be alternating, random, block or graft copolymer.

The sample, which comprises at least a first scale inhibitor, is aliquid sample.

According to one exemplary embodiment scale inhibitor comprises at leastone, preferably two or more ionised groups, more preferably at leastthree ionised groups, even more preferably at least four ionised groups,attached to the scale inhibitor compound structure or polymer/copolymerbackbone. According to another exemplary embodiment scale inhibitorcomprises one or two ionised groups, per at least some of the monomerunits of the scale inhibitor polymer/copolymer. It is not necessary thatall monomer units comprise ionised groups. The ionised groups may beselected from phosphates, phosphonates, carboxylates, sulphonates and/oramines, preferably from carboxylates, sulphonates and/or amines. Aminesmay be primary amines, secondary amines, tertiary amines and/orquaternary amines. Phosphates may be primary phosphates or secondaryphosphates. In case the scale inhibitor comprises two or more ionisedgroups, the ionised groups in the scale inhibitor may all be similar toeach other or they may be different from each other. The scale inhibitormay be anionic, cationic or zwitterionic, preferably anionic.

In exemplary embodiments one or more of the ionised groups of the scaleinhibitor are capable of interacting with the reagents comprisinglanthanide(III) ions. In this context the term “interact” means that theionised groups can react, coordinate and/or chelate with the reagentscomprising lanthanide(III) ions. Especially, the ionised groups of thescale inhibitor can react, coordinate and/or chelate with thelanthanide(III) ions.

According to various embodiments of the invention the scale inhibitor isselected from group comprising polyelectrolyte compounds comprisingcarboxylate and/or phosphonate groups; homopolymers and copolymers ofethylenically unsaturated acid monomers; organophosphonates; andcombinations thereof. The polyelectrolyte compounds may comprise amultiplicity of interactive groups, which can be ionised, for example,carboxylate and/or phosphonate groups. The scale inhibitor may be, forexample, a polycarboxylic acid, such as polyacrylic acid,polymethacrylic acid, polymaleic acid or any of their salts withmonovalent cations. Alternatively the scale inhibitor may be, forexample, maleic anhydride. The scale inhibitor may be a homopolymer or acopolymer of an alpha, beta-ethylenically unsaturated acid monomer suchas acrylic acid or methacrylic acid, a diacid such as maleic acid ormaleic anhydride, itaconic acid, fumaric acid, monoesters of diacidswith alkanols having 1-8 carbon atoms, and/or mixtures thereof. In casethe scale inhibitor is a copolymer, it may be composed of two or moreco-monomers, and the first-co-monomer may be any alpha,beta-ethylenically unsaturated monomer and the second co-monomer may bea non-polar group or monomer, such as styrene or olefinic monomer; or apolar functional group or monomer, such as vinyl acetate, vinylchloride, vinyl alcohol, an alkyl acrylate, vinyl pyridine, vinylpyrrolidone, acrylamide or an acrylamide derivative, etc.; or an ionicfunctional group or monomer, such as styrenesulfonic acid,2-acrylamido-2-methylpropanesulfonic acid (AMPS), vinylsulfonic acid orvinylphosphonic acid. The scale inhibitor may be an organophosphonate,such as amino tris(methylene phosphonic acid), 1-hydroxyethylidene-1,1-diphosphonic acid, diethylenetriamine penta(methylenephosphonic acid) or phosphonobutane-tricarboxylic acid.

The scale inhibitor may have any necessary or desired molecular weight.For example, in an exemplary embodiment, the scale inhibitor may have amolecular weight of from about 500 to about 100 000 Daltons, preferably500 to 100 000 Daltons, more preferably 500-30 000 Daltons, even morepreferably 1000-12 000 Daltons.

In exemplary embodiments, the dosing or concentration of the scaleinhibitor to an aqueous system will be an amount sufficient to produce adesired reduction, control, or inhibition result. In each system, thescale inhibitor may have a predetermined set point, e.g. amount orrange, to achieve a desired effect. The exemplary methods can be used todetect the concentration of the scale inhibitor in the system, so thatthe predetermined set point amount or range can be achieved and/ormaintained. For example, the concentration of the scale inhibitor in aliquid sample, originating for example from an oilfield, oil well orfrom an oil production process, is sometimes in the range of 0.5-200ppm, preferably 1-50 ppm, more preferably 1-10 ppm. The sensitivity ofthe method may be selected so that it can detect the concentration of ascale inhibitor within the effective amount or range. For example, themethod may be configured to directly detect or measure the concentrationof the scale inhibitor in the liquid sample within this range.Alternatively, additional steps may be taken to adapt the method ormodify the sample, e.g., with optional purification and/or dilutionsteps, so that the concentration of the scale inhibitor therein fallswithin the detection limits of the method.

According to one embodiment of the invention, one or more of the scaleinhibitors in a sample interact with a reagent comprisinglanthanide(III) ion, and the resulting interaction product(s) is(are)detected using a time-resolved luminescence technique. An exemplarylanthanide(III) ion is selected from europium, terbium, samarium ordysprosium ions, preferably from europium ion or terbium ion. Even morepreferably the lanthanide(III) ion is europium ion. Exemplary reagentscomprising a lanthanide(III) ion may be a lanthanide(III) salt, such asEuCl₃ or TbCl₃, or a luminescent lanthanide chelate, such as{2,2′,2″,2′″-[(4′-phenyl-2,2′:6′-2″-terpyridine-6,6″-diyl)bis(methylenenitrilo)]tetrakis(acetato)}europium(III)or2,2′,2″,2′″[[4-[(4-phenyl)ethynyl]pyridine-2,6-diyl]bis(methylenenitrilo)]tetrakis-(acetato)europium(III).Preferably, the reagent comprising a lanthanide(III) ion is alanthanide(III) salt, such as EuCl₃ or TbCl₃, more preferablyeuropium(III) salt, such as EuCl₃.

According to another embodiment of the invention it is also possible touse a combination of different reagents with same or differentlanthanide(III) ions. For example, if the sample comprises a pluralityof different scale inhibitors which have different affinity to differentreagents and/or lanthanide(III) ions, it is possible to determine theconcentration of the first scale inhibitor by using a sample signal froma first reagent having a lanthanide(III) ion and the concentration ofthe second scale inhibitor by using a sample signal from a differentsecond reagent having a lanthanide(III) ion.

According to an embodiment it is possible to use a low amount oflanthanide(III) ion for determining the scale inhibitor concentration inthe sample. According to one embodiment the concentration of thelanthanide(III) ion in the sample may be in the range of 0.01-10 mM,preferably 0.01-1 mM, more preferably 0.01 mM-0.1 mM, even morepreferably about 0.01 mM. The lanthanide(III) ion concentration is givenfor the final sample volume for which the time-resolved luminescencemeasurement is performed.

According to the various embodiments, time-resolved luminescencemeasurement can be used to measure the concentration of an individualscale inhibitor, or a plurality thereof, in a liquid sample, and/or tomeasure the combined total concentration of a plurality of scaleinhibitors. In embodiments in which an individual scale inhibitorconcentration is being determined, the reagent comprisinglanthanide(III) ion may be configured to preferentially interact withthe selected individual scale inhibitor being measured, and theinteraction product is detected using time-resolved luminescencemeasurement. In embodiments in which a combined total concentration of aplurality of scale inhibitors is being determined, the reagentcomprising lanthanide(III) ion may be configured to interact with all ofthe plurality of scale inhibitors, and the combined interaction productsare detected using time-resolved luminescence measurement.

Preferably the time-resolved luminescence measurement is time-resolvedfluorescence measurement. In time-resolved fluorescence, the samplecontaining the interaction product(s) of one or more scale inhibitor(s)and one or more reagent(s) comprising lanthanide(III) ion is excited atan excitation wavelength and the fluorescence sample signal is detectedat an emission signal wavelength. An exemplary gate time between theexcitation and emission may be, for example 0.5-800 μs, preferably 1-500μs. The emission signal wavelength is typically longer than theexcitation wavelength.

Excitation wavelength, which is used in the present method, may beselected or determined by studying the excitation maximum in theexcitation spectrum of the formed interaction product of the first scaleinhibitor and the reagent comprising lanthanide(III) ion. For example,the excitation wavelength may be in the range of 200-400 nm and theemission signal wavelength for the sample signal may be about 500-650nm. For example, the excitation wavelength for europium is 340 nm andthe optimum emission signal wavelength 615 nm. Correspondingly, theexcitation wavelength for terbium is 254 nm and the optimum emissionsignal wavelength 545 nm. The excitation spectrum for the interactionproduct of the scale inhibitor and reagent comprising lanthanide(III)ion may be measured prior to starting the determination protocol or theexcitation spectrum may be obtained or estimated from the literature.

It is also possible that the sample further comprises a second scaleinhibitor, optionally a plurality of successive scale inhibitors, andthe method may be used to determine individual concentrations of eachscale inhibitor in the sample. The concentration of a second scaleinhibitor or any successive scale inhibitor in the presence of the firstscale inhibitor may be determined by using time-resolved luminescence orany other suitable analytical technique.

According to one embodiment of the invention the concentration of afirst scale inhibitor is determined by exciting the sample at a firstexcitation wavelength and detecting a first sample signal deriving fromthe lanthanide(III) ion by using time-resolved luminescence measurement,and the concentration of the second scale inhibitor is determined byexciting the sample at a second excitation wavelength and detecting asecond sample signal deriving from the lanthanide(III) ion by usingtime-resolved luminescence measurement. The individual excitationspectrum for each respective interacted scale inhibitor andlanthanide(III) ion may be measured prior to starting the measurement orthe individual excitation spectra may be obtained or estimated from theliterature. For example, the first excitation wavelength may besubstantially the excitation maximum of the first scale inhibitor in thepresence of the reagent comprising lanthanide(III) ion, and the secondexcitation wavelength may be substantially the excitation maximum of thesecond scale inhibitor in the presence of the reagent comprisinglanthanide(III) ion. The reagents for determining the first and thesecond scale inhibitor may be the same or different. The reagents may,for example, comprise different lanthanide(III) ions.

If time-resolved luminescence is being used to measure a plurality ofinteraction products within a sample, the method may be configured inorder to better distinguish the interaction product signals from eachother. For example, it is possible to use different excitationwavelengths, different reagents comprising different lanthanide(III)ions, and/or different signal modifiers, respectively, with one or moreof the interaction products, to help to distinguish the signalsresulting from such interaction products.

In case two or more scale inhibitors are detected by using reagent(s)comprising lanthanide(III) ions and time-resolved luminescencemeasurement, it may be desirable to have a measurable difference betweenthe excitation wavelengths of the first and second and optionally anysuccessive interaction products of scale inhibitors and reagent(s). Forexample, the difference between the first excitation wavelength andsecond excitation wavelength, and optionally any successive excitationwavelength, is at least 10 nm, preferably at least 20 nm, morepreferably at least 25 nm. Depending on the sensitivity of themeasurement device, a greater difference between the excitationwavelengths may make it easier to distinguish the detected samplesignals corresponding to the respective scale inhibitor concentrationsin the sample. According to one embodiment of the invention the samplecomprises three or more different scale inhibitors and the excitation isperformed at three or more excitation wavelengths, respectively. Eachexcitation wavelength may be chosen according to the excitation maximumof each interacted scale inhibitor which is being determined.

According to one embodiment of the invention a signal modifier, whichcomprises a metal ion, may be added to the sample before the excitationof the sample. The signal modifier may be used to modify the samplesignal, e.g. its intensity, or to modify the difference betweenexcitation wavelengths for different scale inhibitors. An exemplarysignal modifier may comprise a metal ion which is selected from a groupcomprising copper, nickel, chromium, iron, gold, silver, cobalt, and anyof their mixtures. Preferably the signal modifier comprises copper(II).It may also possible to modify the effect of the sample matrix to thesample signal by using a signal modifier.

The concentration of a second scale inhibitor, or any successive scaleinhibitor, may alternatively be determined by using any suitableanalytical technique or detection method. Exemplary methods include, forexample, luminescence, time-resolved luminescence, direct fluorescence,absorbance, spectrophotometry, optical rotation measurement, photoncounting, inductively coupled plasma (IPC), high-performance liquidchromatography (HPLC), liquid chromatography-mass spectrometry (LC-MS),size exclusion chromatography, colorimetric methods, NMR, or acombination thereof. For determining individual concentrations for aplurality of different scale inhibitors, respectively, in one sample anypossible combination of the said analytical techniques or detectionmethods may be used. For example, in a sample comprising three scaleinhibitors the concentrations of the first and second scale inhibitorsmay be determined by using reagents comprising lanthanide(III) ions andtime-resolved luminescence measurement and the concentration of thethird scale inhibitor may be determined by using another spectroscopicmeasurement method. Alternatively, one of the scale inhibitors may betagged with a fluorescence tag, such as fluorescein, and the individualconcentration of that tagged scale inhibitor may be determined withdirect fluorescence detection of the tag.

In exemplary embodiments, the sample may be pre-treated before theconcentration of the scale inhibitor is measured. According to oneembodiment of the invention the sample may be purified before theaddition of and/or interaction with the reagent comprisinglanthanide(III) ion for removal of disturbing or interfering substancesand/or compounds. For example, pre-cleaning may help to minimise thebackground noise caused by the components of a liquid sample, e.g.water, system. Exemplary purification methods include, e.g.,centrifugation, size exclusion chromatography, cleaning with solid-phaseextraction (SPE) cartridges, dialysis techniques, extraction methods forremoving hydrocarbons, filtration, microfiltration, ultrafiltration,nanofiltration, membrane centrifugation, and/or other methods used toseparate the polymeric species from smaller compounds, for example othertreatment chemicals or salts. In an embodiment, salt concentration ofthe sample may be reduced or insoluble particles may be removed beforeaddition of the reagent comprising lanthanide(III) ion and thetime-resolved luminescence measurement. In another exemplary embodiment,salt concentration of the sample may be reduced before addition of thereagent comprising lanthanide(III) ion and the performance of thetime-resolved luminescence measurement. In another exemplary embodiment,if the initial concentration of the scale inhibitor in the sample ishigh, e.g. outside of the detection limits of the method, the sample maybe diluted before addition and/or interaction with the reagentcomprising lanthanide(III) ion. Possible diluents are water, one or moreaqueous buffer solutions, or any of their mixtures. In exemplaryembodiments, one or more of the above pretreatment steps may beperformed on a sample before measurement of scale inhibitorconcentration. For example, before measurement the sample may be eitherpurified or diluted, or the sample may be both purified and diluted.

In exemplary embodiments, one or more buffers may be added to the sampleprior to the measurement, to improve the signal-to-noise andsignal-to-background ratio of the detected sample signals. Examples ofthese buffers include, for example, those comprising sulfonic acidderivatives, such as e.g. HEPES(2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonic acid, pK_(a) 7.48),PIPES (1,4-piperazinediethanesulfonic acid, pK_(a) 6.76), MOPS(3-morpholinopropane-1-sulfonic acid, pK_(a) 7.2) and MES(2-(N-morpholino)ethanesulfonic acid, pK_(a) 6.15), HEPES beingpreferred. Further, one preferred buffer is TRIS(2-Amino-2-hydroxymethyl-propane-1,3-diol), especially used as a mixturewith a buffer comprising sulfonic acid derivative, such as HEPES.

According to one embodiment the pH value of the sample is adjusted to asuitable level, for example, in the range between pH 3 and pH 8,preferably in the range from pH 5 to pH 8. Any suitable buffer that doesnot significantly disturb the detection of sample signal, may be used.Exemplary buffers are given above, but other buffers may also be used.

The methods described herein may be automatized or they may be performedmanually. According to one embodiment the method is performed as on-linemeasurement. In an exemplary embodiment, the measurement is preferablyused on-site, such as at an offshore oil platform and provides almostinstant information about on-going production. In some embodiments, themeasurement time is relatively fast, so that for example the totalmeasurement time for analysing one sample from optional purification toobtaining the scale inhibitor concentration value may be less than 15minutes, preferably less than 10 minutes.

The analysis of the scale inhibitor concentrations according to thepresent invention may be performed in any suitable detection or fluidvessel. The fluid vessel may be e.g. a well, a part of a fluidic device,microfluidic chip or a cuvette. The fluid vessel may be selected toprovide a predetermined amount of sample fluid for measurement. Forexample, according to one embodiment of the invention a fixed volume ofthe sample is added to a first fluid vessel, such as cuvette, comprisinga reagent comprising lanthanide(III) ions. The first scale inhibitor inthe sample is allowed to interact with the reagent comprisinglanthanide(III) ions in the first fluid vessel, and the sample isexcited at the first excitation wavelength and the time-resolvedfluorescence signal is detected at the signal wavelength.

In case it is desirable to further measure the concentration of a secondscale inhibitor or any successive scale inhibitor that is in the samesample, the determination may be performed separately, e.g. in parallelor series, by using a plurality of fluid vessels. For example, apredetermined number of fluid vessels may be selected, corresponding tothe number of scale inhibitors to be determined. Each fluid vessel maybe independently configured to determine a respective scale inhibitor.The same or different detection method may be used in each fluid vessel,depending for example on the nature of the scale inhibitor.

For example, according to one embodiment of the invention, the samplemay comprise two or more scale inhibitors, each of which may be measuredaccording to time-resolved fluorescence. The measurement apparatus maycontain a plurality of fluid vessels, and each fluid vessel mayindependently comprise a reagent comprising lanthanide(III) ion, that isselected for the respective scale inhibitor to be determined in suchvessel. A fixed volume of the sample may be introduced to each fluidvessel, and the scale inhibitors may be allowed to interact with thereagents in the respective fluid vessels. In each vessel the sample isexcited at an excitation wavelength and the time-resolved fluorescencesignal is detected at the signal wavelength. As a result, each fluidvessel may provide information related to the concentration of adifferent scale inhibitor in the sample.

In embodiments in which a plurality of fluid vessels are used, thereagent comprising lanthanide(III) may be the same or different in thedifferent fluid vessels. The excitation wavelength of the sample in thesecond fluid vessel or successive fluid vessels may be different thanthe excitation wavelength of the sample in the first fluid vessel.

For example, according to another embodiment the sample comprises afirst scale inhibitor and a second scale inhibitor, and theconcentration of each is determined, respectively, in first and secondfluid vessels. In the first fluid vessel, the first scale inhibitor isallowed to interact with a known amount of a reagent comprising, forexample, terbium(III) ion. In the second vessel the second scaleinhibitor is allowed to interact with a known amount of a reagentcomprising, for example, europium(III) ion. The individualconcentrations of the first and the second scale inhibitors aredetermined on basis of the measured time-resolved fluorescence samplesignals of terbium and europium reacted with the first and second scaleinhibitor, respectively.

According to another embodiment of the invention the sample may comprisefirst, second and third scale inhibitors, of which the third scaleinhibitor is tagged with a chromophore. A fixed volume of the sample isadded to three fluid vessels. The first and second vessels comprisereagents with lanthanide(III) ions. In the first fluid vessel the firstscale inhibitor is allowed to interact with a first reagent comprisinglanthanide(III) ion, the sample is excited at the first excitationwavelength, which is specific for the first scale inhibitor and thetime-resolved fluorescence signal is detected at the signal wavelength,the first sample signal corresponding to the concentration of the firstscale inhibitor. In the second fluid vessel the second scale inhibitoris allowed to interact with a second reagent comprising lanthanide(III)ion, the sample is excited at the second excitation wavelength, which isspecific for the second scale inhibitor and the time-resolvedfluorescence signal is detected at the signal wavelength, the secondsample signal corresponding to the concentration of the second scaleinhibitor. In the third fluid vessel the concentration of the thirdscale inhibitor is obtained by measurement of the inherent absorbance ofthe sample. Since only the third scale inhibitor comprises achromophore, the measured absorbance is proportional to theconcentration of the third scale inhibitor in the sample. In case thethird scale inhibitor is tagged with a fluorescent tag, thedetermination of the concentration of the third scale inhibitor may beperformed on basis of a direct fluorescence signal of the tag. Using theresults obtained from the three fluid vessels, one may determine therespective concentrations of the first, second, and third scaleinhibitors in the sample.

According to another embodiment of the invention the method may beperformed in a single fluid vessel, even if the sample comprises aplurality of scale inhibitors. For example, the sample may comprise afirst, a second and a third scale inhibitor, of which the third scaleinhibitor is tagged with a fluorophore. A fixed volume of the sample isadded to a fluid vessel, e.g. cuvette, comprising a reagent comprisinglanthanide(III) ions and the scale inhibitors are allowed to interactwith the reagent comprising lanthanide(III) ions. The concentration ofthe first scale inhibitor in the sample is obtained when the sample isexcited at the first excitation wavelength specific for the first scaleinhibitor and the time-resolved fluorescence sample signal is detectedat the signal wavelength, this first sample signal corresponding to theconcentration of the first scale inhibitor. The concentration of thesecond scale inhibitor is obtained when the sample is excited at thesecond excitation wavelength, which is specific for the second scaleinhibitor and the second time-resolved fluorescence sample signal isdetected at the signal wavelength, this second sample signalcorresponding to the concentration of the second scale inhibitor. Theconcentration of the third scale inhibitor is obtained by directmeasurement of the specific fluorescence signal of the fluorophore.

According to one embodiment of the invention the sample comprises two ormore scale inhibitors, whereby the detected sample signal deriving fromthe lanthanide(III) ion at the signal wavelength by using time-resolvedluminescence measurement corresponds to the total concentration of thetwo or more scale inhibitors in the sample. In this case the samplecomprises a plurality of scale inhibitors, which absorb excitationenergy at the same wavelength, and the sample signal, in fact,correlates with the total concentration of all scale inhibitors in thesample, which are excited at the same wavelength.

The method according to the invention is quantitative, i.e. the signalswhich are obtained for the sample or for first, second or eachsuccessive scale inhibitor, correspond to the total concentration of allscale inhibitors or to the individual concentrations first, second oreach successive scale inhibitor.

For determining the concentration of a scale inhibitor, a standard curveor standard point may be prepared before performing the determinationmethod. The concentration of the scale inhibitor may be calculated onbasis of the obtained sample signal by using the predetermined standardcurve or the standard point. Alternatively the measurement instrumentmay be pre-calibrated. In case the sample comprises a plurality of scaleinhibitors, a standard curve or standard point may be prepared for eachscale inhibitor to be determined.

EXPERIMENTAL

Some embodiments of the invention are described more closely in thefollowing non-limiting examples.

Examples employ scale inhibitor polymers, which are sulphonatedpolycarboxylates, i.e. copolymers comprising allylsulphonate- and maleicanhydride based monomers in 50/50 molar ratio. The molecular weight ofthe copolymers is between 1500 and 12 000 Da. In the examples, thefollowing scale inhibitor polymers, given in Table 1, are referenced.

TABLE 1 Scale inhibitor polymers of Examples 1 and 2 Polymer DescriptionP1 Sulphonated polycarboxylate P2 Sulphonated polycarboxylate with afluorescent moiety P40 Sulphonated polycarboxylate with a phosphorousmoiety

Example 1

Concentration of a scale inhibitor polymer is measured in the Example 1.

First, 1450 μL of artificial brine solution comprising 600 mM NaCl, 7 mMMgCl₂*6H₂O, 15 mM CaCl₂*2H₂O, 3 mM KCl and 0.5 mM BaCl₂, in filtratedand deionized (MilliQ) water was added to a cuvette (Fischer brand Macrocuvette, Polystyrene, FB55143). Thereafter, 250 μL of 10 μM of EuCl₃ ina 5 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)-NaOHbuffer, pH 7.6, and 800 μL of artificial brine solution as abovecontaining varying concentrations of scale inhibitor polymer was addedto the cuvette. The time-resolved luminescence signal was measured witha luminescence reader (Aqsens Oy, Turku, Finland) using excitationwavelength of 395 nm, emission wavelength of 614 nm, delay time of 1 μsand measurement window 950 μs. The concentration of the single scaleinhibitor polymer P1 was measured as seen in FIG. 1 (square, black). Thetotal concentration of three mixed scale inhibitor polymers P1, P2 andP40 (FIG. 1, circle, grey) containing equal concentration of eachpolymer in the mixture results in an equal time-resolved luminescencesignal level at each given concentration. This example shows that singlescale inhibitor polymer is measured with efficiency. In FIG. 1:X-axis=polymer concentration (ppm), Y-axis=time-resolved luminescencesignal.

Example 2

Total concentration of scale inhibitor polymers are measured in theExample 2.

Experiment set-up was identical to that of Example 1. The totalconcentration of a mixture of three scale inhibitor polymers P1, P2 andP40 was measured as shown in FIG. 1 (circle, black). The samplecontaining equal concentration of each scale inhibitor polymer in themixture resulted in an equal time-resolved luminescence signal level ateach given concentration. The example shows that a mixture of scaleinhibitor polymers is measured with equal efficiency. In FIG. 1:X-axis=polymer concentration (ppm), Y-axis=time-resolved luminescencesignal.

It is apparent to a person skilled in the art that the invention is notlimited exclusively to the examples described above, but that theinvention can vary within the scope of the claims presented below.

1. A method for determining a scale inhibitor concentration in a samplecomprising at least a first scale inhibitor, which is a syntheticorganic compound comprising at least one ionised group, the methodcomprising optionally diluting and/or purifying the sample, allowing theat least first scale inhibitor in the sample to interact with a reagentcomprising a lanthanide(III) ion, exciting the sample at a firstexcitation wavelength and detecting a sample signal deriving from thelanthanide(III) ion at a signal wavelength by using time-resolvedluminescence measurement, and determining the concentration of the atleast first scale inhibitor in the sample by using the detected samplesignal, wherein the scale inhibitor concentration is determined in anindustrial water system or industrial water system sample.
 2. The methodaccording to claim 1, wherein the reagent comprising a lanthanide(III)ion is a lanthanide(III) salt or a luminescent lanthanide chelate. 3.The method according to claim 1 wherein the lanthanide(III) ion isselected from europium, terbium, samarium or dysprosium ions.
 4. Themethod according to claim 1, wherein the concentration of thelanthanide(III) ion in the sample is in the range of 0.01-10 mM.
 5. Themethod according to claim 1, wherein the scale inhibitor comprises twoor more ionised groups, the ionised groups being selected fromphosphates, phosphonates, carboxylates, sulphonates, or amines.
 6. Themethod according to claim 5, wherein the scale inhibitor is selectedfrom the group consisting of polyelectrolyte compounds comprisingcarboxylate and/or phosphonate groups; homopolymers and copolymers ofethylenically unsaturated acid monomers; organophosphonates; andcombinations thereof.
 7. The method according to claim 1, wherein theconcentration of the scale inhibitor in the sample is in the range of0.5-200 ppm.
 8. The method according to claim 1, wherein the samplecomprises two or more scale inhibitors, whereby the detected samplesignal deriving from the lanthanide(III) ion at the signal wavelength byusing time-resolved luminescence measurement corresponds to the totalconcentration of the two or more scale inhibitors in the sample.
 9. Themethod according to claim 1, wherein the sample comprises a second scaleinhibitor, optionally a plurality of successive scale inhibitors, andthe individual concentrations of each scale inhibitor in the sample aredetermined.
 10. The method according to claim 9, wherein theconcentration of the second scale inhibitor, or any successive scaleinhibitor, is determined by using luminescence, time-resolvedluminescence, direct fluorescence, absorbance, spectrophotometry,optical rotation measurement, photon counting, inductively coupledplasma (IPC), high-performance liquid chromatography (HPLC), liquidchromatography-mass spectrometry (LC-MS), size exclusion chromatography,colorimetric methods, NMR, or a combination thereof.
 11. The methodaccording to claim 9, wherein the concentration of the second scaleinhibitor is determined by exciting the sample at a second excitationwavelength and detecting a second sample signal deriving from thelanthanide(III) ion by using time-resolved luminescence measurement. 12.The method according to claim 11, wherein the difference between thefirst excitation wavelength and second excitation wavelength is at least10 nm.
 13. The method according to, claim 1, wherein a signal modifiercomprising a metal ion, is added to the sample before the excitation ofthe sample.
 14. The method according to claim 13, wherein the signalmodifier comprises a metal ion which is selected from the groupconsisting of copper, nickel, chromium, iron, gold, silver, cobalt, andany of their mixtures.
 15. The method according to, claim 1, wherein thetime-resolved luminescence measurement is time-resolved fluorescencemeasurement.
 16. The method according to, claim 1, wherein the sample ispurified by using a purification method selected from centrifugation,size exclusion chromatography, cleaning with solid-phase extraction(SPE) cartridges, dialysis techniques, extraction methods for removinghydrocarbons, filtration, microfiltration, ultrafiltration,nanofiltration, membrane centrifugation and any combinations thereof.17. The method according to claim 1, wherein a pH value of the sample isadjusted to a level in the range between pH 3 and pH
 8. 18. (canceled)19. The method according to claim 1, wherein the industrial water systemis selected from, cooling tower water systems including open,recirculating, closed and once-through systems; boilers and boiler watersystems; mineral process waters including mineral washing, flotation andbenefaction; paper mill digesters, washers, bleach plants and whitewater systems; black liquor evaporators in the pulp industry; gasscrubbers and air washers; continuous casting processes in themetallurgical industry; air conditioning and refrigeration systems;industrial and petroleum process water; indirect contact cooling andheating water; water reclamation and purification systems; membranefiltration water systems; food processing streams; or waste treatmentsystems.
 20. The method according to claim 19, wherein the industrialwater system is food processing stream selected from meat, vegetable,sugar beet, sugar cane, grain, poultry, fruit or soybean processingstreams.
 21. The method according to claim 1, wherein the industrialwater system is selected from the group consisting of clarifiers;liquid-solid applications; municipal sewage treatment; and industrial ormunicipal water systems.
 22. The method according to claim 2, whereinthe reagent is salt selected from EuCl₃, TbCl₃,{2,2′,2″,2′″-[(4″-phenyl-2,2″:6″-2″-terpyridine-6,6″-diyl)bis-(methylenenitrilo)]-tetrakis(acetato)}europium(III)or2,2′,2″,2′″[[4-[(4-phenyl)-ethynyl]pyridine-2,6-diyl]bis(methylenenitrilo)]tetrakis(acetato)europium(III).23. The method according to claim 1, wherein the concentration of thelanthanide(III) ion in the sample is in the range of 0.01-1 mM.
 24. Themethod according to claim 7, wherein the concentration of the scaleinhibitor in the sample is in the range of 1-50 ppm
 25. The methodaccording to claim 17, wherein a pH value of the sample is adjusted to alevel in the range from pH 5 to pH
 8. 26. The method according to claim1, wherein it is performed as on-line measurement.